JPH0648776B2 - Active filter - Google Patents

Description

The present invention relates to an active filter integrated into a semiconductor integrated circuit, and more particularly to a center frequency of the filter.
It is related to the ones whose adjustment of ₀ is easy.

[Prior Art] FIG. 4 shows a biquad circuit, which is an example of a conventional active filter, and constitutes a low-pass filter. The transfer function T (S) of a general low-pass filter is
It is expressed by the following functional expression.

[Where ω ₀ is an angular frequency, s is a complex variable, Q is a coil loss coefficient, and H is a gain coefficient] The relationship between the input voltage V ₁ and the output voltage V ₂ of the biquad circuit in FIG. When expressed by (S), it is expressed by the following equation.

Assuming that the coefficients of the terms in the equations (1) and (2) are equal, the angular frequency ω ₀ and the figure of merit Q are expressed by the following relational expressions.

_{ω 0 = (1 / R 02} R 04 C 01 C 02) 1/2 ......... (3) Q = (R 01 2 C 01 / R 02 R 04 C 02 1/2 ...... (4) (3) As is clear from the equation (4), in order to change the center frequency ₀ and the figure of merit Q, it is necessary to change the circuit constants of the resistors R _{01 to} R ₀₄ or the capacitors C ₀₁ and C _02. Therefore, it is necessary to adjust the filter characteristics by changing the circuit constants of the external components, and therefore, in such an active filter, a large number of components such as operational amplifiers, resistors and capacitors are mixed and integrated to form a circuit. Is usually formed.

[Problems to be Solved by the Invention]

The conventional active filter has the drawback that the number of parts is large and the cost is high, and the center frequency of the filter is _0.
When adjusting the or performance index Q, there is the complexity of having to adjust the components having different circuit constants on the printed circuit board according to the desired filter characteristics, and it is not easy to finely adjust the bandwidth of the filter. .

The present invention has been made based on the above-mentioned problems, and its main object is to provide an active filter in which the number of parts is reduced by being made into a semiconductor integrated circuit.

Another object of the present invention is to provide an active filter having a large variable range in which the center frequency ₀ can be easily adjusted while keeping the filter performance index Q constant.

[Means for solving the problem]

The active filter of the present invention is a first operational amplifier circuit comprising a first differential amplifier having an emitter negative feedback resistor and a first output circuit for converting the output of the first differential amplifier into a single output. A first integrating circuit in which a first capacitor is connected to the output terminal of the first operational amplifier circuit, a second differential amplifier including an emitter negative feedback resistor, and a second differential amplifier. A second output circuit that converts the output to a single output
A second integrator circuit in which a second capacitor is connected to the output terminals of the operational amplifier circuit and the second operational amplifier circuit, the output from the first integrator circuit being supplied. from it, the center frequency ₀ by comprising a variable current source circuit to the first and current respectively variable current between the first and supplied to the second differential amplifier is supplied to the second output circuit It is variable.

[Action]

The active filter of the present invention comprises a current I _Y supplied to the first and second differential amplifiers of their input stages that form the first and second integrator circuits and their output stages. The current I _X supplied to the first and second output circuits is supplied via the current source circuits to change the center frequency ₀ .

〔Example〕

FIG. 1 shows an embodiment of an active filter showing a second-order low pass filter according to the present invention.

In FIG. 1, the input terminal 1 is connected to the non-inverted terminal of the operational amplifier A ₁ , and its output terminal is connected to the capacitor C ₁ and the inversion terminal of the operational amplifier A ₂ .
The output terminal of the operational amplifier A ₂ is connected to the inverting terminal of the operational amplifier A _1, A ₂ together constitute a negative feedback circuit, it is connected to the capacitor C ₂ and the output terminal 2. Operational amplifier A
₁ constitutes an integration circuit, Similarly, the operational amplifier A ₂ to form an integrating circuit by the capacitor C ₂ to form a second-order low-pass filtered by a capacitor C _1.

The current source circuit 3 is a circuit that controls the current supplied to the operational amplifiers A ₁ and A ₂ , and is composed of current mirror circuits 4 to 6, a constant current source circuit 7, and a variable current source circuit 8. .

The current mirror circuit 5 is composed of transistors Q _{4 to} Q ₆ and resistors R _{4 to} R _6, and the constant current source circuit 7 is connected to a connection point between the collector of the transistor Q _{4 and} the base of the transistor Q ₅ . . The current mirror circuit 4 also has a similar structure, and is composed of transistors Q _{1 to} Q ₃ and resistors R _{1 to} R _4, and a variable current source is provided at a connection point between the collector of the transistor Q _{1 and} the base of the transistor Q _2. Circuit 8
Are connected, and the collector of the transistor Q ₃ is connected to the connection point between the collector of the transistor Q _{4 and} the base of the transistor Q ₅ . On the other hand, the collector of the transistor _{Q 6,} the collector and the transistor of the transistor _{Q 7 Q 8}
The current mirror circuit 6 is formed by the transistor Q ₉ and the resistor R ₇ which are connected to the base of the current mirror circuit 6.

FIG. 2 shows a more specific embodiment of the active filter of FIG. In the figure, diodes D ₁ and D are provided to the collectors of the transistors Q ₁₁ and Q ₁₂ , respectively.
The cathode of ₂ is connected, and the emitter negative feedback resistor R ₁₀ is connected between the emitters of the transistors Q ₁₁ and Q ₁₂ . The cathode of the diode D ₃ is connected to the commonly connected anodes of the diodes D ₁ and D ₂ , and the anode thereof is connected to the voltage source. The base of the transistor Q ₁₃ is connected to the connection point between the diode D ₂ and the transistor Q _12, and the base of the transistor Q ₁₄ is the diode D ₁
Is connected to the connection point of the transistor Q ₁₁ and the transistor Q ₁₁ . The emitter of the transistor Q _13, Q ₁₄ are connected in common to the collector of the transistor Q _19. The collectors of the transistors Q ₁₃ and Q ₁₄ are transistors Q _{25 to} Q respectively.
_It is connected to the current mirror circuit 9 composed of ₂₇ and the resistor R _12, and the capacitor C ₁ is connected to the collectors of the transistors Q ₁₄ and Q ₂₇ to form an integrating circuit.

Transistors Q ₁₁ , Q ₁₂ , diodes D ₁ , D ₂ ,
D ₃ , the resistor R ₁₀ , and the transistors Q ₂₁ , Q ₂₂ forming the current mirror circuit 6 constitute a first differential amplifier, and the transistors Q ₁₃ , Q ₁₄ , Q ₁₉ , the resistor R ₁₄ , and the current mirror circuit. 9 is a first output for converting the output of the first differential amplifier into a single output
, And the operational amplifier A ₁ is composed of these two circuit parts.

The operational amplifier A ₂ has the same configuration as the operational amplifier A _1, and the output terminal thereof is connected to the capacitor C ₂ to form an integrating circuit. The description of the configuration of the operational amplifier A ₂ is omitted. The collectors of the transistors Q ₁₄ and Q ₂₇ of the operational amplifier A ₁ are connected to the base of the transistor Q ₁₅ of the operational amplifier A ₂ , and the base of the transistor Q _{12 is} the base of the transistor Q ₁₆ and the transistors Q ₁₈ and Q ₃₀ , respectively. Is connected to the collector of and is connected to the output terminal 2.

Differential pair transistor Q at the input stage of operational amplifiers A ₁ and A ₂
The bases of the transistors Q _{21 to} Q ₂₄ connected to the emitters of ₁₁ , Q ₁₂ and Q ₁₅ , Q ₁₆ are commonly connected, and are connected to the collector of the transistor Q ₉ of the current source circuit 3 to form the current mirror circuit 6. ing. The common connection point of the bases of the transistors Q ₁₉ and Q ₂₀ in the output stage of the operational amplifiers A ₁ and A ₂ is connected to the bases of the transistors Q ₁ and Q ₃ of the current mirror circuit 4, and the resistors R _{1 to} R ₃ are connected. And transistor Q ₂
Together with this, it constitutes a current mirror circuit. The collector of the transistor Q ₃ of the current mirror circuit 4 is connected to the transistor Q ₄
It is connected to the connection point between the collector of and the constant current source circuit 7.

The current mirror circuits 4, 5, 6, 9, and 10 are not limited to the current mirror circuit shown in FIG. 2, and various forms can be implemented.

Next, the operation will be described based on the integrating circuit constituting the active filter in FIG. The input voltage is V ₁ , the output voltage is V _0, and the bias currents of the input stage differential amplifier and the output circuit are I _Y and I _X , respectively. The current of the signal component is i
₀ and the current flowing through the resistor R ₁₀ is i _a , the AC voltage V ₁ applied between the bases of the transistors Q ₁₁ and Q ₁₂ is
The following equation holds.

_{i a = V 1 / R 10} ...... (5) diodes _D 1, _{D 2} of the cathode potential, and when the base voltage difference of the transistor Q _13, Q ₁₄ and _{V a} is expressed by the following equation.

_{V a = V T 1n (I} Y + i a) / I S1 -V T 1n (I Y -i a) / I S1 = V T 1n (I Y + i a) / (I Y -i a) ...... ( 6) V _a = V _T 1n (I _X + i ₀ ) / I _S2 −V _T 1n (I _X −i ₀ ) / I _S2 = V _T 1 n (I _X + i ₀ ) / (I _X −i ₀ ) ... (7) [where I _S1 is the saturation current of the diodes D ₁ and D ₂ and I _S2 is the saturation current between the base and emitter of the transistors Q ₁₃ and Q ₁₄ , and they are equal to each other. V
_T is a thermal voltage. ] From equations (6) and (7), the following equation holds.

_{1n (I Y + i a)} / (I Y -i a) = 1n (I X + i 0) / (I X -i 0) (I Y + i a) / (I Y -i a) = (I X + i ₀ ) / (I _X −i ₀ ) By rearranging the above equation and substituting the equation (5), the signal current i ₀ is expressed by the following equation.

i ₀ = I _X i _a / I _Y = I _X V ₁ / I _Y R ₁₀ (8) On the other hand, the output voltage V ₀ of the integrator is V ₀ = i ₀ / sC (however, s = jω) By substituting the equation (8) into the above equation, V ₀ = I _X V ₁ / I _Y R ₁₀ sC (9) Therefore, the following equation holds from the equation (9).

Moreover, the mutual conductance gm is calculated from the equation (9). Then, the transfer function T (S) of the integrating circuit is expressed by the following equations from the equations (10) and (11).

T (S) = V ₀ / V ₁ = gm / sC = 1 / srC [where r = 1 / gm ] (12) The resistance component r of the operational amplifier A ₁ has a relationship of r = 1 / gm with the transconductance gm of the integrator circuit, and the transfer function T (S) of the integrator has bias currents I _Y , I It is clear that it depends on the function of _{X 1} and resistance R ₁₀ . That is, the integrator circuit shows that the transfer function T (S) of the integrator fluctuates by controlling the bias currents I _Y and I _X.

Next, the figure of merit Q and the center frequency ₀ of the active filter shown in FIG. 1 will be described, and the current source circuit 3 for controlling the figure of merit will be described.

The resistance component r of the operational amplifiers A ₁ and A ₂ forming the integrating circuit
(R = 1 / gm) is set as r ₁ and r ₂ , respectively, and the capacitor C
_{Assuming 1} and C ₂ , the transfer function of each integrator has the following relationship.

Qω ₀ / s = 1 / sr ₁ C ₁ ω ₀ / sQ = 1 / sr ₂ C ₂ Further, when the above equation is arranged, it is expressed as the following equation.

Qω ₀ = 1 / r ₁ C ₁ (13) ω ₀ / Q = 1 / r ₂ C ₂ (14) Also, 1 / r ₁ = gm ₁ = I _X / I _Y · R ₁₀ 1 / r ₂ = gm ₂ = I _X / I _Y · R ₁₀ By rearranging the above formula, the following formula is obtained.

r ₁ = I _Y · R ₁₀ / I _X ...... (15) r ₂ = I _Y · R ₁₀ / I _X …… (16) Therefore, the figure of merit Q is calculated from the equations (13) and (14) as follows. Then, by substituting the equations (15) and (16) into the equation, Qω ₀ / (ω ₀ / Q) = Q ² = r ₂ C ₂ / r ₁ C ₁ = (C ₂ I _Y R ₁₀ / I _X ) / (C ₁ I _Y R ₁₀ / I _X ), that is, the performance index Q is expressed by the following equation.

Q = (C ₂ / C ₁ ) ^1/2 (17) As is clear from the equation (17), the performance index Q is the current source circuit 3
It is proved that even if the currents I _X and I _Y supplied from the above are held at constant values even if they are varied.

Further, the center frequency ω ₀ is obtained from the equations (13) and (14) as follows, and by substituting the equations (15) and (16), Qω ₀ · ω ₀ / Q = ω ₀ ² = 1 / r ₁ C ₁ r ₂ C ₂ = I _X ² I _Y R ₁₀ ² C ₁ C ₂ , and the center frequency ω ₀ is represented by the following formula from this formula.

ω ₀ = 1 / I _Y R ₁₀ · (I _X ² / C ₁ C ₂ ) ^1/2 …… (1
8) Therefore, the center frequency ₀ is expressed by the following expression from the expression (18).

The current I supplied to the differential amplifiers of the operational amplifiers A ₁ and A ₂
The relationship between _X and the current I _Y supplied to the output circuit is set by the current source circuit 3, the current supplied from the current mirror circuit 4 is A · _IX , and the current mirror circuit 6 outputs the operational amplifier A ₁ , the current supplied to the input stage of the _{a 2} is a constant current of the constant current circuit 7 When a _{I N,} the current of _{I Y = I N -A · I} X is supplied. Therefore, the center frequency ₀ is (19)
From the formula, it is expressed as follows.

By controlling the current I _X supplied from the variable current source circuit 8 of the current source circuit 3 from the equation (20), it is possible to increase the current I _X without changing Q as described in FIG. By changing the center frequency ₀ , the pass band can be changed as shown in (a), (b), and (c).

〔effect〕

INDUSTRIAL APPLICABILITY The active filter of the present invention is suitable for a semiconductor integrated circuit, can reduce the number of external parts as compared with the conventional one, keep the figure of merit Q constant, and reduce only the center frequency _{0 of the} filter characteristic. Easy to adjust.

Further, since the active filter of the present invention is formed of a semiconductor integrated circuit, it is possible to supply an inexpensive active filter because the number of externally attached parts can be reduced, and it is possible to miniaturize the active filter. .

Further, the active filter of the present invention has a center frequency
_It also has the effect that the variable currents I _X and I _Y supplied with the _zero adjustment can be achieved by a relatively small value.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showing an embodiment of an active filter according to the present invention, and FIG. 2 is a circuit diagram showing an embodiment in which FIG. 1 is more concrete. FIG. 3 shows the active and
FIG. 4 is a diagram showing a filter characteristic of a filter, and FIG. 4 is a circuit diagram showing an example of a conventional biquad circuit. 1: input terminal, 2: output terminal, 3: current source circuit, 4,
5, 6, 9, 10: current mirror circuit, 7: constant current source circuit, 8: variable current source, A ₁ , A ₂ : operational amplifier, C ₁ ,
C _2: capacitors,

Claims (2)

[Claims] 1. A first differential amplifier having an emitter negative feedback resistor and a first differential amplifier for converting the outputs of the first differential amplifier into a single output.
A first integrator circuit in which a first capacitor is connected to the output terminal of a first operational amplifier circuit composed of the above output circuit, a second differential amplifier including an emitter negative feedback resistor, and the second difference amplifier. A second integrating circuit in which a second capacitor is connected to the output terminal of a second operational amplifying circuit which is a second output circuit for converting the output of the dynamic amplifier into a single output; and the first integrating circuit Is supplied to the second differential amplifier on the input side of the second operational amplifier circuit, and the current supplied to the first and second differential amplifiers and the first and second output circuits is An active filter comprising a variable current source circuit. 2. A first differential amplifier having an emitter negative feedback resistor, and a first differential amplifier for converting the output of the first differential amplifier into a single output.
A first integrator circuit in which a first capacitor is connected to the output terminal of a first operational amplifier circuit composed of the above output circuit, a second differential amplifier including an emitter negative feedback resistor, and the second difference amplifier. A second integrating circuit in which a second capacitor is connected to the output terminal of a second operational amplifying circuit which is a second output circuit for converting the output of the dynamic amplifier into a single output; and the first integrating circuit Is supplied to a second differential amplifier on the input side of the second operational amplifier circuit, and current is supplied to the first and second differential amplifiers and the first and second output circuits. A current source circuit for supplying a current value at which the sum of the increment of the current supplied to the first and second output circuits and the current supplied to the first and second differential amplifiers becomes constant. Active filter characterized by the fact.